STABILIZATION AND LOCALIZATION OF XIST RNA ARE CONTROLLED BY SEPARATEMECHANISMS AND ARE NOT SUFFICIENT FOR X-INACTIVATION

These studies address whether XIST RNA is properly localized to the X chromosome in somatic cells where human XIST expression is reactivated , but fails to result in X inactivation (Tinker, A.V., and C.J. Brown. 1998. Nucl. Acids Res. 26:2935-2940). Despite a nuclear RNA accumulat ion of normal abundance and stability, XIST RNA does not localize in r eactivants or in naturally inactive human X chromosomes in mouse/human hybrid cells. The XIST transcripts are fully stabilized despite their inability to localize, and hence XIST RNA localization can be uncoupl ed from stabilization, indicating that these are separate steps contro lled by distinct mechanisms. Mouse Xist RNA tightly localized to an ac tive X chromosome, demonstrating for the first time that the active X chromosome in somatic cells is competent to associate with Xist RNA. T hese results imply that species-specific factors, present even in matu re, somatic cells that do not normally express Xist, are necessary for localization. When Xist RNA is properly localized to an active mouse X chromosome, X inactivation does not result. Therefore, there is not a strict correlation between Xist localization and chromatin inactivat ion. Moreover, expression, stabilization, and localization of Xist RNA are not sufficient for X inactivation. We hypothesize that chromosoma l association of XIST RNA may initiate subsequent developmental events required to enact transcriptional silencing.